In recent years, an Insulated Gate Bipolar Transistor (IGBT) capable of being driven at high current is used as a switching element.
FIG. 4 is an example of a cross-sectional structure of a typical IGBT element. In this IGBT element 100, functions of a bipolar transistor and a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) are combined. In this structure, n layer 102 serving as a base of the bipolar transistor is formed on a p-type substrate 101 serving as a collector of the bipolar transistor, and a p+ layer 103, an n+ layer 104, a gate oxide 105 which is a part of the MOSFET, and a gate 106 are formed on the surface side (upper side in FIG. 4). An emitter electrode 107 is connected to the p+ layer 103 and n+ layer 104, and an insulating layer 108 for preventing short-circuit between the emitter electrode 107 and gate 106 is formed. The IGBT 100 is turned ON when the gate voltage thereof is increased not less than a predetermined threshold voltage. At this time, holes are injected from the p-type substrate 101 to n layer 102 to cause conductivity modulation, allowing high current to flow, that is, reducing on-resistance. Therefore, by increasing the amount of holes to be injected, on-resistance (on-voltage) can be reduced. Actually, inmost cases, a plurality of the IGBT elements 100 each having the above configuration are formed on a single substrate and are connected in parallel so as to reduce particularly the on-resistance.
When the IGBT element 100 is turned OFF, the gate voltage thereof is decreased to less than the predetermined threshold voltage. At this time, current continues to flow until the holes existing in the n layer 102 at the ON-time disappear. That is, as the electrons disappear when being recombined with electrons, the IGBT 100 is not completely turned OFF until the holes disappear. Therefore, in order to increase the switching speed of the IGBT, it is necessary to reduce the time (lifetime of holes) taken for the holes to disappear.
Therefore, there is proposed a structure for reducing the lifetime of holes in the n layer 102. For example, Patent Document 1 discloses a technique that forms, in the n layer, a defect layer in which the lifetime of the holes is reduced by ion implantation. Further, Patent Document 2 discloses a technique that forms the defect layer on the p-type substrate side. By using these technique, the switching performance of the IGBT element 100 has been improved.